How much should it cost to flush the heater core or is it something else? on 2002 Audi A4 Quattro

I noticed this summer that my air conditioner didn't seem to work. Never really got cold. Now, that it's winter, I only get heat on lowest fan setting blowing on windshield and from left vent. Cold air blows at higher fan settings if all vents are activated. Always blows cold on the floor.

Asked by for the 2002 Audi A4 Quattro
could have a heater door actuator problem